Videoconferencing systems are conventionally adjusted to employ both circuit switched and packet switched networks. Thus, great efforts have been made to achieve interoperability between circuit switched and packet switched terminals, allowing interactions and communication without loss of quality or introduction of delay. A common standard for multimedia communications over circuit switched networks such as ISDN is the H.320 standard from the International Telecommunications Union (ITU). H.320 is an umbrella standard for defining the different protocol layers. H.221 is the framing protocol of H.320. The purpose of this recommendation is to define a frame structure for audiovisual teleservices.
The H.323 standard is the corresponding umbrella recommendation for packet switched networks. Such networks are pervasive on many corporate terminals and include TCP/IP and IPX (Inter network packet exchange) over Ethernet, Fast Ethernet and Token Ring network technologies. The H.323 standard provides a foundation for audio, video, and data communications across IP-based networks, including the Internet. Multimedia products and applications complying with the H.323 standard are interoperable, can communicate with each other, and thus are compatible. The H.323 standard or protocol is also made out of many sub standards, one of which is the H.245 standard. The H.245 standard defines the control protocol part of the H.323 standard.
Systems that use different protocols can make video conferencing calls through an H.320/H.323 gateway, which performs the task of “translating” between H.320 and H.323. A typical scenario is where an enterprise, because of cost and management, implements IP as the network to use internally, where they have control of the bandwidth resources available, but uses ISDN externally, where they will not be guaranteed the quality of service on the IP network that they require. Hence, they use a gateway for external calls.
A gateway provides a connection between the respective IP and circuit switched sides of the communication path between endpoints in a multimedia conference. Seen from the endpoints residing at the IP side, the endpoints at the circuit switched side are virtually being converted to IP endpoints, and vice versa. The main tasks of the gateway consequently are to translate and re-pack the data stream across the networks in real-time. The packets transmitted from the H.323 endpoints are temporarily stored in a buffer before they are fetched and arranged in H.320 frames of a fixed size.
A video conferencing call setup from an H.320-based system to an H.323-based system through a gateway typically includes the following steps:
The H.320-based system (A) dials an ISDN-number defined for the gateway.
The gateway accepts the call, and connects the first ISDN B-channel to use.
A BONDING™ procedure is performed, including negotiation of which bandwidth to use for the call, and resolving of which ISDN-numbers A should use to dial the remaining B-channels for the call.
A dials the remaining B-channels to use. The gateway accepts these incoming channels.
An in band communication channel is set up according to H.221 exchanging audio, video and possibly other capabilities.
When the video channel is open, a still image may be encoded in the gateway, and transmitted to A. The image may include text requesting the user of system A to enter the other party's extension (H.323 alias). A voice message may also be presented.
A user of A enters the extension, which may be transmitted to the gateway using DTMF (dual tone multi-frequency) in the audio channel, or through H.320's TCS-4 signal.
When the gateway has received the full extension, the gateway will try to establish an H.323 call to the endpoint registered with this extension (B).
The gateway then initiate a H.323-setup of a call to the called party B by means of RAS-messages (Registration, Admission and Status), Q.931 messages and H.245 messages with the bandwidth used for the H.320 side of the call already initiated. An example of a signal flow at H.323-setup is illustrated in FIG. 1. Normally, the setup is supported by a gatekeeper at the called part (B) side of the connection, i.a. for translating aliases to IP addresses. The setup is also finished by an exchange of the capability information i.a. indicating the bandwidth capability of the endpoint.
When the H.323 call is established, the still image presented to A is removed and the audio and video is repacked and forwarded from A to B and from B to A.
However, if the gateway is unable to set up the call to B on the requested bandwidth (because of lack of resources on B, or in the network), the calls are connected on different bandwidth. A H.320 system and a H.323 system connected through a gateway are not able to use end-to-end flow control. It is only possible for the gateway to send flow control messages to the H.323 system. This bandwidth imbalance is for many reasons an unwanted situation.
A problem also occurs when the H.320 system transmits video at a higher rate than the H.323 system can handle, the H.323 system may fail to handle the call. This may occur if the call is initiated from the H.320 side and the call rate on the H.323 side does not end up being greater or equal to the rate of the H.320 side.
Further, the H.320 call will not utilize its full bandwidth, as the gateway will have to transmit idle pattern to adjust to the lower bandwidth from the H.323 call. This leads to higher ISDN-costs and unnecessary use of resources.
When operating on different bandwidths in the same call, some sort of transcoding has to be performed to adjust the bandwidth used for video data from the H.320 call to the H.323 call. Transcoding uses more resources in the gateway than conventional gateway re-packing, and additionally, full bandwidth of the H.320 call will not be utilized.